1. Field of the Invention
The present invention belongs to the technical field of a resin coated metal sheet, specifically, the technical field of a resin coated metal sheet having an epoxy resin layer on a galvanized metal sheet, particularly a coated steel sheet for automobiles which enables the omission of part of a coating step in the production of an automobile and the omission of the filling of a grease into a bent portion or the like.
2. Description of the Related Art
JP-A 11-5269 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a resin coated metal sheet having a resin coating layer on a galvanized metal sheet or aluminum plated metal sheet.
Although a resin, a curing agent and conductive pigment contained in a coating are set forth in claims of JP-A 11-5269, corrosion preventing additives are merely enumerated in the section “Detailed Description of the Invention”.
Expressions for the thickness “t” (μm) of a coating film and the content “P” (wt %) of the conductive pigment in the coating film are given in claims of JP-A 11-5269. These expressions are the following expressions (1) and (2). In these expressions (1) and (2), when t=7, 39≦P≦71.P≧7.14t−10.71  (1)P≦7.14t+21.43  (2)
Epoxy resin, urethane resin, acrylic resin, polyester resin and modified resins thereof are disclosed as the resin. Isocyanate compounds and/or alkyl etherified amino formaldehyde resin (alkylated melamine resin) are disclosed as the curing agent.
JP-A 2000-522923 discloses a coating composition (coating film) for covering an amorphous metal.
Types and amounts of a resin, curing agent, rust-proof pigment and conductive pigment are defined in claims of JP-A 2000-522923.
Epoxy resin (including epoxy resin having an epoxy group such as dimer acid modified epoxy resin)+blocked polyurethane resin are disclosed as the resin. Guanidine, substituted guanidine, substituted urine, cyclic tertiary amine and mixtures thereof are disclosed as the curing agent. A silicate compound (0 to 15 wt %) is disclosed as the rust-proof pigment. Zinc, aluminum, graphite and/or molybdenum disulfide, carbon black and iron phosphide (40 to 70 wt %) are disclosed as the conductive pigment. The amount of the solvent is 0 to 30 wt %.
The patent JP-A 11-5269 discloses the relationship among the thickness of the coating film, the amount of the conductive pigment, weldability, formability and adhesion but not corrosion resistance which is an important evaluation item in examples at all and attaches importance to weldability and formability. Therefore, it is unknown whether the resin coated metal sheet of the above patent document 1 has excellent corrosion resistance or not.
Meanwhile, JP-A 2001-522923 mainly describes corrosion resistance and fails to disclose weldability and formability. Therefore, it is unknown whether the coating composition of the above patent document has excellent weldability and corrosion resistance.
Consequently, it is unknown whether what are disclosed by the above patent documents are excellent in all of weldability, formability and corrosion resistance and it is possible that they are not excellent in any one of them.